Digitally controlled, refreshed displays are generally implemented by two different methods. Displays can be generated on the screen of a cathode ray tube by using a scan method similar to TV where the picture is generated by moving the intensity producing electron beam horizontally across the cathode ray tube screen at slightly different vertical positions. Several hundred horizontal scans are made per second to prevent the appearance of flicker. Graphic displays on the other hand make use of the ability to digitally specify a position on the cathode ray tube screen and then move the intensity producing electron beam in the manner required to generate the desired symbol or image. A given display may consist of many symbols and/or figures, all of which have to be generated or refreshed several times a second (typically 50 times per second) in order for the image to appear to be constantly illuminated with no flicker.
In order to refresh the display, a memory device is usually provided that contains the digital information necessary to continually regenerate the display. The memory is organized into words each containing a specified number of binary bits that are indicative of an image or portion thereof. A display controller accesses the code words in the memory and interprets the bit configurations to perform a specified display function, much as a digital computer accesses a memory to retrieve instructions that are to be executed. Typical display controller functions include position operations indicating an X and Y location on the cathode ray tube screen; image operations indicating a particular image to be generated at a previously specified X and Y location on the cathode ray tube screen; and line generation operations indicating the direction and length of a line to be generated starting at a previously specified location. Display generation, accordingly, consists of accessing memory words, interpreting the words, executing the specified operations, detecting the of a generation of an image, and refreshing the display at a constant rate to provide a constant intensity to the observer.
In order to make a display "operator interactive" means are provided to allow the operator to indicate a specific cathode ray tube screen location to computing means. Track balls, encoders, and joysticks provide this function by allowing the operator to specify the location on the cathode ray tube screen by the position of a cursor controlled by the operator. After the operator has positioned the cursor at the desired location, a separate switch is activated to indicate that the desired screen location is the position of the cursor. A ligh pen performs the same function directly without the use of a cursor and contains a sensor that is sensitive to the wave length of light emanated from the cathode ray tube screen. Since the cathode ray tube screen emits light from a location only for a short period immediately after the location is traced by the electron beam under the control of the display controller, the output from a light pen held at an illuminated point on the cathode ray tube screen will be present only when the point is being traced by the electron beam. Since the display controller maintains the X and Y position of the electron beam on the cathode ray tube screen during the time the beam is activated to produce the illumination, the light pen output can cause the X and Y position of the electron beam on the cathode ray tube screen indicated by the display controller to be made available to the computing means controlling the display system.
The output from the light pen, however, is delayed by the response time of the light radiating element, such as phospher, of the cathode ray tube screen and the response time of the light pen sensor. As is set forth in U.S. Pat. No. 3,543,240, the contents of which are incorporated herein by reference, there are various other circumstances when the then X and Y positions of the tracing electron beam provided by the display controller do not correspond to the position of the traced display intended to be identified by the light pen output. Accordingly, the X and Y positions of the traced display provided to the computing means by the display controller do not always define the exact location at which the light pen is pointed.
The system described in U.S. Pat. No. 3,543,240 identifies the location at which the light pen is pointed by stepping a counter each time a portion of the traced image is generated by means of the associated code word. The count in the counter corresponds to the address in the refresh memory of the segment of the image then being traced. The address identifying count remains in the counter once the corresponding display segment is identified by a light pen actuation until the computing means is ready to receive the count thereby identifying the light pen selected display segment to the computing means. Although the light pen detection system described in this patent overcomes many deficiencies of the prior art, it does have the disadvantage that each segment of each displayed image must have an address code associated therewith rather than having the flexibility of an identification code identifying an entire symbol that is displayed or separate identification codes for identifying one or more segments of the displayed symbol. Also, since the counter must be successively stepped, the same address cannot be utilized for various segments of the displayed symbols when the computing means response is identical for the segments identified. Further, the light pen detection system described in this patent does not permit the light pen to be disabled whenever the portion of the display that is not to be interactive with the light pen is being traced to reduce input errors to the computing means. Such a capability also allows one image in close proximity, or overlapping, with another image to be light pen distinguishable as long as only the pertinent image is enabled for interaction with the light pen.
Accordingly, one object of this invention is to provide an improved light pen interactive graphic display system.
Another object of this invention is to provide a graphic display system wherein the position of the light pen identified displayed image, or portion thereof, is accurately determined.
A further object of the present invention is to provide a light pen interactive graphic display system wherein the light pen is disabled when the portion of the display not meant to be interactive with the light pen is being traced.
Still another object of this invention is to provide a light pen interactive graphic display system wherein an entire displayed image or any number of segments thereof are identified by a single identification code.
An additional object of this invention is to provide a light pen interactive graphic display system wherein a single identification code identifies one or more displayed images or, alternatively, unique identification codes are provided for at least some of the segments of the displayed images.